The present invention relates to a zoom lens system having a high zoom ratio which is made up of a reduced number of lenses, and is best suited for use on lens shutter cameras having an unlimited back focal distance.
So far, many proposals have been made for a two-unit zoom type comprising a front unit of positive refracting power and a rear unit of negative refracting power. This type, because of being made up of a minimum number of units in view of zoom lens design, has a great merit of being simplified in terms of barrel structure and driving mechanism.
Among such a two-unit type of zoom systems there is known a lens system comprising a reduced number of lenses, as typically disclosed in JP-A 3-127008. As an example, this publication refers to a system made up of a front unit comprising a negative lens and a positive lens and a rear unit comprising a positive lens and a negative lens; four lenses in all. Correction for aberrations is then achieved by use of many aspheric surfaces. However, this lens system costs much, because an aspheric lens made of glass material is needed for the negative lens of the first unit for the purpose of making correction for chromatic aberration in the front unit. Another example is directed to a lens system made up of a reduced number of, i.e., 3, lenses in order to achieve cost reduction. However, this lens system has an increased F-number and is not well corrected for aberrations; it is of low commercial value.
To overcome the problems mentioned above, some systems are proposed in JP-A 5-113537, JP-A 5-188292 and JP-A 5-224122. For cost reduction, the first lenses are all constructed from plastic material, and designed to have small refracting power for the purpose of reducing adverse influences due to changes inherent in plastics, which depend on temperature and humidity. For correction for chromatic aberration, the second lenses are each formed of a cemented lens element. In other words, the total number of lenses is about 5.
A system analogous to those disclosed in the three publications referred to above has also been proposed, as disclosed in JP-A 5-188293. However, this system has a first lens of small refracting power yet made of glass; that is, it fails to achieve any cost reduction.
In addition, all the prior systems mentioned above have a zoom ratio as low as about 2 or about 2.4.